(1) Field of the Invention
This invention relates to a hydrogen production apparatus and a hydrogen production method for producing hydrogen by performing partial oxidization and steam reforming on a hydrocarbon fuel such as natural gas, methanol, or naphtha, and also relates to a fuel cell system for generating electricity using hydrogen produced with the hydrogen production apparatus or the hydrogen production method.
(2) Description of the Prior Art
What is called steam reforming method is widely used in hydrogen production apparatuses which produce hydrogen and supply the produced hydrogen to fuel cells as the fuel. In the steam reforming method, a fuel such as natural gas, methanol, or naphtha is mixed with steam, then the mixture is reformed to a hydrogen-rich gas in the presence of a catalyst. Chemical Formula 1 below shows the reaction occurring in such steam reforming processes, the formula dealing with a case of methane. The reforming reaction is usually induced at a high temperature in a range of about 700-800.degree. C.
Chemical Formula 1 EQU CH.sub.4 +H.sub.2 O.fwdarw.3H.sub.2 +CO
The resultant reformed gas of the above process includes an appreciable amount of carbon monoxide (several and ten percent). However, carbon monoxide reduces the catalysis capability. Therefore, before the reformed gas is supplied to fuel cells, carbon monoxide in the reformed gas is generally converted to carbon dioxide. Chemical Formula 2 below shows the CO shift reaction which occurs in a shift converter including a catalyst for the CO shift reaction. The shift converter operates at a temperature of around 180-300.degree. C. lower than reformers.
Chemical Formula 2 EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
Incidentally, such steam reforming apparatuses may be used at home or in the open air. Accordingly, it is desirable that the steam reforming apparatuses are compact.
Phosphoric-acid type fuel cell systems have a high cell operating temperature of around 200.degree. C. At such a high cell operating temperature, the coolant water used for cooling the fuel cell is evaporated. The steam generated in the evaporation can be used for the steam reforming. On the other hand, polymer electrolyte fuel cell systems have a low cell operating temperature of around 80.degree. C. At such a low cell operating temperature, steam cannot be directly obtained from the coolant water.
Conventional polymer electrolyte fuel cell systems disclosed in Japanese Laid-Open Patent Nos.1-264903 and 5-186201 are provided with a reaction tank which includes a two-layered, cylinder-shaped container filled with a catalyst for the reforming. The reaction tank receives a supply of fuel and steam while it is heated by a burner.
Japanese Laid-Open Patent No.7-335238 discloses an apparatus that first mixes a material with air then partially oxidizes the mixture using a catalyst for partial oxidization. The apparatus then mixes the high-temperature fuel gas with steam then reforms the mixed gas in the presence of a catalyst. It is possible for this apparatus to use the heat, which is generated in the partial oxidization, for the steam reforming reaction.
However, such hydrogen production apparatuses based on steam reforming methods require, outside of the apparatuses, a steam supply source such as a boiler. As a result, one problem of the hydrogen production apparatuses is that they cannot run in an environment where such a steam supply source is not available.
Concerning the mixture ratio of fuel and steam, it is considered as appropriate that the steam/carbon ratio (S/C ratio) is in a range of 2.5-3.5. Conventional techniques generally use pressure controllers or flow-rate controllers to control the pressure or flow rate of steam so that the S/C ratio stays in an appropriate range. However, it is desirable to use more convenient apparatuses than conventional ones to control the S/C ratio.